Leptospirosis, caused by the spirochete Leptospira interrogans and close relatives of that genus, is a significant zoonotic disease affecting humans throughout the United Sates and many other parts of the world. These bacteria infect humans and other animals by penetrating mucous membranes or conjunctiva, or through breaks in the skin. Bacteria then disseminate throughout the host via the bloodstream, followed by invasion of other tissues. Leptospirosis in humans can lead to a range of debilitating symptoms, and, not infrequently, death. Understanding the mechanisms by which leptospires are able to initiate and establish infection can direct development of improved diagnostics and preventative/treatment therapies. Infectious leptospires are very resistant to their hosts'alternative pathway of complement-mediated killing. Our studies found that L. interrogans can bind the host complement regulator factor H to its outer surface, a strategy adopted by many other pathogens to avoid killing by host complement. During infection, L. interrogans invades and colonizes kidneys and other host organs, apparently facilitated by the spirochete's abilities to interact with host extracellular matrix (ECM) components. We discovered that L. interrogans carries six separate but similar genes, each of which encodes a protein that can bind factor H and/or ECM proteins. All six leptospiral proteins share predicted structural and some functional similarities with mammalian endostatin, and have been designated LenA, LenB, LenC, LenD, LenE and LenF (Len = Leptospiral ENdostatin-like protein). We hypothesize that Len proteins enable L. interrogans to both resist killing by host complement and to interact with host ECM. As a corollary to our central hypothesis, we predict that all infectious leptospires produce one or more Len proteins to facilitate mammalian infection, and that infected humans and other animals produce antibodies against those Len proteins. To test our hypothesis, we will (1) perform functional characterization of interactions between Len proteins and their host ligands and (2) evaluate the practicality of using Len proteins for serological diagnosis of leptospirosis patients. PUBLIC HEALTH RELEVANCE: The causative agents of leptospirosis, Leptospira interrogans and closely-related members of that genus, are highly invasive pathogens that can establish persistent infections of immunocompetent individuals. The aim of this project is to characterize a family of six leptospiral proteins we found to interact with human immune system regulators and extracellular matrix components, and may therefore play important roles in human leptospirosis.